Lithium-garnet solid electrolyte composite, tape articles, and methods thereof

ABSTRACT

A composite ceramic including: a lithium garnet major phase; and a grain growth inhibitor minor phase, as defined herein. Also disclosed is a method of making composite ceramic, pellets and tapes thereof, a solid electrolyte, and an electrochemical device including the solid electrolyte, as defined herein.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/953,050, filed on Apr. 13, 2018, which claims the benefit of priorityto Chinese Patent Application No. 201710248253.4, filed on Apr. 17,2017, the contents of which are relied upon and incorporated herein byreference in their entireties as if fully set forth below.

BACKGROUND

The disclosure relates generally to lithium ion solid electrolyteceramic compositions, and more particularly to the Li-garnet oxides, andto methods of making the lithium ion solid electrolyte ceramiccompositions.

SUMMARY

In embodiments, the disclosure provides a composite Li-garnet ceramicelectrolyte of the formula Li_(7-x)La₃(Zr_(2-x), M_(x))O₁₂-SA, where Mis, for example, Al, Ga, In, Si, Ge, Sn, Sb, Bi, Sc, Y, Ti, Hf, V, Nb,Ta, W, or a mixture thereof; “SA” refers to a second additive (SA) oxideselected from the group MgO, CaO, ZrO₂, HfO₂, or mixtures thereof,present in a specified wt % or mol % based on the total amount of theceramic; and x is greater than 0 and less than 1 (i.e., 0<x<1).Exemplary Li-garnet composite oxide ceramics contain the secondadditive, for example, in an amount of up to 9 wt % such as from 1 to 9wt % based on the total weight of the composite.

The second additive is stable in combination with the cubic Li-garnetoxides, and can exist as an individual or discrete minor phase duringthe formation process. However, the elements of the second additive canalso be a component in the garnet oxides. Although not limited bytheory, the second additive is believed to enhance the mechanicalproperties of the composite by improving the uniformity of themicrostructure and decreasing the grain size of the garnet oxide majorphase. Although not bound by theory, the second additive is believed torestrain abnormal grain growth in the ceramic.

In embodiments, the disclosed Li-garnet composite ceramic can be usefulas an electrolyte in, for example, an energy storage article.

In embodiments, the disclosure provides a method of forming a Li-garnetcomposite electrolyte comprising: forming a mixture of a cubic Li-garnetoxide and a second additive by a wet-milling process; preheating themixture at a low calcination temperature; milling the mixture by adry-milling process; compacting the mixture; and sintering the compactat a sintering temperature, wherein the resulting composite Li-garnetoxide contains the second additive in an amount up to 9 wt. % such asfrom 1 to 9 wt % based on the total weight of the composite.

BRIEF DESCRIPTION OF THE DRAWINGS

In embodiments of the disclosure:

FIG. 1 is a graph showing XRD patterns of Control Example 1 and Examples2 to 10.

FIG. 2 is a graph showing AC impedance at room temperature correspondingto Control Example 1 and Examples 2 to 10, respectively.

FIG. 3 is a graph showing the Arrhenius plots and fitting resultscorresponding to Control Example 1 and Examples 2 to 10, respectively.

FIG. 4 is a graph showing the relationships between MgO content inweight percentage (wt %) and the total conductivity, and MgO content inweight percent and the activation energy (Ea) corresponding to ControlExample 1 and Examples 2 to 10.

FIG. 5 is a graph showing the relationships between MgO content (in wt%) of the disclosed ceramics and the theoretical density (left axis),the practical or real density (left axis), and the relative density(right axis) in the Control Example 1 and Examples 2 to 10.

FIG. 6 is a graph showing the relationships between MgO content (in wt%) of the ceramics and the fracture stress (i.e., strength; left axis),and the Vickers hardness (right axis) corresponding to Control Example 1and Examples 2 to 10, respectively.

FIGS. 7A to 7J show scanning electron microscope (SEM) images ofpristine sintered samples in cross-section corresponding to ControlExample 1 and Examples 2 to 10, respectively.

FIGS. 8A to 8J show SEM images of chemical corrosion samples incross-section corresponding to Control Example 1 and Examples 2 to 10,respectively.

FIGS. 9A to 9I show energy dispersive spectroscopy (EDS) images of MgOdistributions of pristine samples corresponding to Examples 2 to 10,respectively.

FIGS. 10A to 10J show back scattering electrons (BSE) images of pristinesamples corresponding to Control Example 1 and Examples 2 to 10,respectively.

FIGS. 11A and 11B, respectively, show particle size distributions by vol% for a Ga—Nb doped garnet composition of the formulaLi_(6.75)La_(2.8)Ga_(0.2)Zr_(1.75)Nb_(0.25)O₁₂ (“HF 110”), and for aTa-doped garnet composition of the formulaLi_(6.5)La₃Zr_(1.5)Ta_(0.5)O₁₂ (“HQ 110”).

FIG. 12 shows an image of an exemplary disclosed translucent orsemi-transparent tape after sintering in air.

FIGS. 13A to 13C, respectively, show selected microstructural aspects oflithium garnet tapes: surface of tape (13A); polished surface (13B); andfracture surface (13C).

FIG. 14 shows a schematic of an exemplary sintering apparatus.

FIG. 15 shows an image of an exemplary disk shape of Ta doped garnettape after sintering in air, illustrating its flatness, translucentappearance, and uniform color properties.

FIGS. 16A to 16D show SEM cross-section images that demonstrate that thegarnet tapes are sintered dense and have small grain sizes.

FIG. 17 is a graph of a representative sintering profile of a disclosedgarnet tape.

DETAILED DESCRIPTION

Various embodiments of the disclosure will be described in detail withreference to drawings, if any. Reference to various embodiments does notlimit the scope of the invention, which is limited only by the scope ofthe claims attached hereto. Additionally, any examples set forth in thisspecification are not limiting and merely set forth some of the manypossible embodiments of the claimed invention.

In embodiments, the disclosed method of making and using provide one ormore advantageous features or aspects, including for example asdiscussed below. Features or aspects recited in any of the claims aregenerally applicable to all facets of the invention. Any recited singleor multiple feature or aspect in any one claim can be combined orpermuted with any other recited feature or aspect in any other claim orclaims.

Definitions

“Major phase” or like terms or phrases refer to a physical presence of alithium garnet in greater than 50% by weight, by volume, by mols, orlike measures in the composition.

“Minor phase” or like terms or phrases refer to a physical presence of agrain growth inhibitor in less than 50% by weight, by volume, by mols,or like measures in the composition.

“SA,” “second additive,” “second phase additive,” “second phase additiveoxide,” “phase additive oxide,” “additive oxide,” “additive,” or liketerms refer to an additive oxide that produces a minor phase or secondminor phase within the major phase (i.e., the first phase) when includedin the disclosed compositions.

“Include,” “includes,” or like terms means encompassing but not limitedto, that is, inclusive and not exclusive.

“About” modifying, for example, the quantity of an ingredient in acomposition, concentrations, volumes, process temperature, process time,yields, flow rates, pressures, viscosities, and like values, and rangesthereof, or a dimension of a component, and like values, and rangesthereof, employed in describing the embodiments of the disclosure,refers to variation in the numerical quantity that can occur, forexample: through typical measuring and handling procedures used forpreparing materials, compositions, composites, concentrates, componentparts, articles of manufacture, or use formulations; through inadvertenterror in these procedures; through differences in the manufacture,source, or purity of starting materials or ingredients used to carry outthe methods; and like considerations. The term “about” also encompassesamounts that differ due to aging of a composition or formulation with aparticular initial concentration or mixture, and amounts that differ dueto mixing or processing a composition or formulation with a particularinitial concentration or mixture.

“Optional” or “optionally” means that the subsequently described eventor circumstance can or cannot occur, and that the description includesinstances where the event or circumstance occurs and instances where itdoes not.

The indefinite article “a” or “an” and its corresponding definitearticle “the” as used herein means at least one, or one or more, unlessspecified otherwise.

Abbreviations, which are well known to one of ordinary skill in the art,may be used (e.g., “h” or “hrs” for hour or hours, “g” or “gm” forgram(s), “mL” for milliliters, and “rt” for room temperature, “nm” fornanometers, and like abbreviations).

Specific and preferred values disclosed for components, ingredients,additives, dimensions, conditions, times, and like aspects, and rangesthereof, are for illustration only; they do not exclude other definedvalues or other values within defined ranges. The compositions,articles, and methods of the disclosure can include any value or anycombination of the values, specific values, more specific values, andpreferred values described herein, including explicit or implicitintermediate values and ranges.

High energy density lithium ion batteries (LIBs) are promisingelectrochemical energy storage devices. LIBs play a significant role inthe development of portable electronic devices, electric vehicles, andload-leveling applications. Compared to conventional commercial LIBs,new LIB systems such as Li-air and Li—S batteries (see Ji X, et al.,Advances in Li—S batteries, Journal of Materials Chemistry, 2010,20(44): 9821-9826.) are under development for the next-generationbatteries to provide higher energy density and reduce cost. In aqueousLi-Air batteries, the configuration involves a solid electrolytemembrane to separate the lithium anode from the air catholyte. For Li—Sbatteries, an internal “shuttle” phenomenon caused by the solubility ofthe long chain polysulfide in the organic electrolyte can decrease theactivity mass utilization and reduce the columbic efficiency. Novelconfigurations that use a solid electrolyte membrane might be able tosolve these problems and significantly prolong cycling without capacityfading. An all-solid-state lithium secondary battery that uses a solidelectrolyte membrane may also be a future candidate for its improvedsafety. In the configurations of various applications, the solidelectrolyte functions as the lithium ion conductor, the separatinglayer, the protector, and the underlay. Significant to development ofthe above projected new-generation batteries is a solid electrolytemembrane having excellent high relative density, high ionicconductivity, good chemical stability, and good mechanical properties. Asignificant challenge in the formation of such membranes via traditionalceramic routes is the inability to sinter suitable starting materials toa sufficient strength to form a membrane while producing the requisiteconductivity and economy.

Recently, a novel class of lithium-stuffed garnet oxides has shownpromising performance as solid electrolytes due to high ionicconductivity, chemical stability vs. lithium metal and a wideelectrochemical window (see Murugan, R., et al., Fast lithium ionconduction in garnet-type Li₇La₃Zr₂O₁₂, Angewandte Chemie-InternationalEdition, 2007, 46(41): 7778-7781). A number of elements such as Al, Ga,Y, Si, Ge, Nb, Ta, and Te have been doped into garnet to further improvethe ionic conductivity by stabilizing the garnet cubic structure andmuch progress has been achieved. For example, the Ta-doped Li₇La₃Zr₂O₁₂(LLZO) displays a favorable lithium ionic conductivity of 8×10⁻⁴ S/cm,which is significantly higher than the undoped LLZO (see Li, Y. T., etal., Optimizing Li⁺ conductivity in a garnet framework, Journal ofMaterials Chemistry, 2012, 22(30): 15357-15361). At the same time,design and fabrication of the all solid-state lithium batteries is beingexplored with garnet-type electrolytes (see Ohta, S., et al.,Electrochemical performance of an all-solid-state lithium ion batterywith garnet-type oxide electrolyte, Journal of Power Sources, 2012, 202:332-335; Kotobuki, M, et al., Compatibility of Li₇La₃Zr₂O₁₂ SolidElectrolyte to All-Solid-State Battery Using Li Metal Anode, Journal ofthe Electrochemical Society, 2010, 157(10): A1076-A1079; Ohta, S., etal., All-solid-state lithium ion battery using garnet-type oxide andLi₃BO₃ solid electrolytes fabricated by screen-printing, Journal ofPower Sources, 2013, 238(0): 53-56; Ohta, S., et al., Co-sinterablelithium garnet-type oxide electrolyte with cathode for all-solid-statelithium ion battery, Journal of Power Sources, 2014, 265: 40-44). CubicLi-garnet oxides have shown promise as membrane materials in novel LIBs.However, it has not been possible for the garnet oxides to meet therequisite conductivity and mechanical strength required for practicalapplication.

In the field of ceramics it is generally recognized that the strength ofbrittle polycrystalline materials can be affected by factors such asporosity and grain size (see Knudsen, F., Dependence of mechanicalstrength of brittle polycrystalline specimens on porosity and grainsize, Journal of the American Ceramic Society, 1959, 42(8): 376-387).Abnormal grain growth can significantly decrease the mechanicalproperties. One approach to reducing or limiting grain growth is to adda compatible second phase which can pin grain boundaries (see Lange, F.F., et al., Hindrance of Grain Growth In Al₂O₃ by ZrO₂ Inclusions(1984), Journal of the American Ceramic Society, 67 (3), pp. 164-168).But in the garnet-type electrolytes, such an approach has not beenimplemented. Furukawa has added a 0.05 to 1 wt. % of an Al additive and0.05 to 1 wt. % of a Mg additive to restrain the growth of garnet grains(see Furukawa, M., Solid electrolyte ceramic material and productionmethod therefor, WO2013128759). The addition of a Mg containing additivecan be accomplished by a Mg containing powder or a Mg containingcrucible. Since the addition of Mg or Al is very small, the majority ofthe additive that is dissolved within the garnet material and,consequently, the product is not a composite of the added oxide withLi-garnet solid electrolyte. The fracture strength of the garnet oxidesmentioned in the embodiments of WO2013128759 was lower than that of theLi-garnet solid electrolyte composite ceramics of the presentdisclosure.

In embodiments, the disclosure provides a lithium ion solid electrolyteceramic composition, such as a Li-garnet oxide. The solid electrolytecomposition can further include one or more additives, which additivecan improve the uniformity of the ceramic microstructure and can enhancethe mechanical properties of the ceramic. As used herein “uniformity ofthe ceramic microstructure” refers to the distribution of grain sizes.The occurrence of abnormally large grains, which can have a detrimentaleffect on mechanical properties, can be eliminated. For example, themaximum grain size measured for a population of grains representing atleast 5% of the total grains should not exceed the average grain size bymore than a multiple of 20.

In embodiments, the present disclosure provides a composite ceramiccomprising:

a lithium garnet major phase; and

a grain growth inhibitor minor phase, wherein lithium garnet major phaseis Li_(6.4)La₃Zr_(1.4)Ta_(0.6)O₁₂ and comprises between 91 to 99 wt % ofthe composite ceramic, and the grain growth inhibitor minor phase is MgOand comprises between 1 to 9 wt % of the composite ceramic.

In embodiments, the present disclosure provides a composite electrolytecomprising: a lithium garnet ceramic, having a lithium garnet majorphase and a grain growth inhibitor minor phase, of the formula:

Li_(7-x)La₃(Zr_(2-x),M_(x))O₁₂—SA,

where

M is selected from the group Al, Ga, In, Si, Ge, Sn, Sb, Bi, Sc, Y, Ti,Hf, V, Nb, Ta, W, or a mixture thereof, and

“SA” comprises a second additive oxide selected from the group MgO, CaO,ZrO₂, HfO₂, or a mixture thereof, present in from 1 to 9 wt % based onthe total amount of the ceramic; and x is greater than 0 and lessthan 1. In embodiments, the lithium garnet major phase can be, forexample, Li_(6.4)La₃Zr_(1.4)Ta_(0.6)O₁₂ and can comprise between 91 to99 wt % of the composite ceramic, and the grain growth inhibitor minorphase is MgO and can comprise from 1 to 9 wt % based on the total weightof the composite ceramic

In embodiments, the ceramic can have an average grain size of, forexample, from 3 to 7 microns.

In embodiments, the ceramic can have a mechanical strength of, forexample, from 100 to 180 MPa, such as 125 to 145 MPa, includingintermediate values and ranges.

In embodiments, the ceramic has an ion conductivity, for example, offrom 1×10⁻⁴ S/cm to 6×10⁻⁴ S/cm such as 1×10⁻⁴ S/cm to 3×10⁻⁴ S/cm,including intermediate values and ranges, see for example Table 1.

In embodiments, the sintering can be accomplished in air at, forexample, from 1000 to 1300° C. such as from 1000 to 1250° C., includingintermediate values and ranges.

In embodiments, the density of the ceramic is at least 95 to 98% of atheoretical maximum density of the ceramic.

In embodiments, the present disclosure provides a ceramic electrolytecomprising at least the above mentioned composite ceramic.

The present disclosure is advantaged is several aspects, including forexample:

In embodiments, the microstructure of the membrane having an addedsecond phase (i.e., “additive”) is more uniform in the distribution ofthe grain size compared to a similar membrane without the added secondphase.

In embodiments, the grain size of the disclosed garnet electrolytehaving a second additive phase is smaller compared to a similar materialwithout the second additive.

In embodiments, the mechanical strength of the disclosed garnetelectrolyte is improved having the additive as a result of the abovemembrane microstructure, and the above smaller grain size of the garnetelectrolyte.

In embodiments, the disclosed garnet electrolytes that include apreferred second additive, no or minimal loss of conductivity issuffered compared to the same garnet without the second additive or freeof the second additive.

In embodiments, the present disclosure provides a method of making theabove mentioned composite ceramic, comprising:

a first mixing of inorganic source materials to form a mixture,including a lithium source compound, and other suitable inorganic sourcematerials to make the desired garnet composition;

a first milling the mixture to reduce the particle size, as definedherein, of the precursors;

calcining the milled mixture to form a garnet oxide at from 600 to 1200°C.;

a second mixing of the first milled and calcined garnet oxide and asecond additive to provide a second mixture;

a second milling of the second mixture to reduce the particle size, asdefined herein, of constituents of the second mixture; and

compacting the second milled, second mixture into a compact; and

sintering the compact at from 850 to 1300° C.

In embodiments, the lithium source compound can be present, for example,in a stoichiometric excess.

In embodiments, the second additive can be, for example, MgO in from 1to 9 wt % based on the total weight of the composite.

In embodiments, the sintering can be accomplished, for example, in air,in an inert atmosphere, or first in air then in an inert atmosphere.

In embodiments, the incorporation of certain amounts of a secondadditive in the composite ceramic assists in restraining or inhibitingthe growth of the abnormal garnet grains during high temperaturesintering, which inhibition significantly improves the microstructureuniformity of the garnet ceramics.

In embodiments, the crystal grain size of garnet electrolyte can bedecreased by, for example, introducing a certain amount of the secondadditive.

In embodiments, the method of making the Li-garnet oxides can select therelative density of the Li-garnet oxide product, for example, therelative density can be increased by the incorporation of certainamounts of a second additive such as MgO.

In embodiments, incorporation of a certain amount of the second additivein the Li-garnet oxide can enhance the mechanical properties whilemaintaining a high total ion conductivity and a low activation energy(Ea) in a composite ceramic electrolyte.

The disclosed composite ceramic electrolytes comprise the garnet phaseand the individual second additive phase.

The disclosed garnet-type oxides including the second additive canimprove the fabrication and long-term performance in applications thatrely on solid-state ionic conductors.

In embodiments, the disclosure provides a method of making a tape of theabovementioned composite electrolyte, comprising:

thoroughly mixing, wet or dry, a lithium garnet batch including oxideprecursors to form a batch mixture powder;

calcining the batch mixture powder in a platinum container to form acalcined powder;

milling the calcined powder to form a milled powder;

tape casting the milled powder to form a green tape; and

sintering the green tape to form the tape of the composite electrolyte.

In embodiments, the method can further comprise classifying the milledpowder to a mono-modal distribution having a particle size of from 0.3to 0.7 microns.

In embodiments, a lithium garnet membrane has been produced by a tapecasting method and subsequent sintering. The aqueous slip systemprovides a method of making tapes of a lithium garnet powder. It ispossible to stack and laminate the tapes to form a thin sheet. Sinteredtape can be produced in air or in an inert atmosphere, e.g., Ar or N₂.In embodiments, the tape can have a thickness from 20 to 300 microns,and a high density of about 95%. The tape can be flat, translucent, andhermetic. The ion conductivity of the tape can be, for example, higherthan 10⁻⁴ S/cm whether sintered in air or in an inert atmosphere.

In embodiments, the present disclosure provides a method of making athin membrane of lithium garnet electrolyte by tape casting from anaqueous slip system.

In embodiments, a lithium garnet solid electrolyte has been produced bytape casting. The tape casted electrolyte can have, for example, athickness of 80 to 300 microns and a planer dimension larger than, e.g.,a 1 inch (2.54 cm) square or a 1 inch diameter circular shape.

In embodiments, the present disclosure provides methods of making alithium garnet solid membrane as a solid electrolyte, including lithiumgarnet powder fabrication, garnet setter formation, tape castingprocess, non-touch cover powder placement, platinum protection, andsintering conditions.

In embodiments, the lithium garnet powder can be produced by a solidstate reaction. A stoichiometric batch can be thoroughly mixed by drymixing or wet mixing prior to calcination. The final composition has anominal lithium garnet chemical formulaLi_(7-x)La_(3-y)Zr_(2-x)A_(x)B_(y)O₁₂ with A⁺, B⁺, or both doping, whichcan be, for example, Al, Nb oxide, Ga oxide, Ta oxide, and like dopants.The A⁺ or B⁺ doping can promote the stability of lithium cubic garnetphase at low temperature or during application conditioning. The cubicgarnet phase is called for to provide high ion conductivity. It isdesirable for the lithium garnet powder to contain a lithium garnetcubic phase at greater than 90 wt % with an average particle size D₅₀less than 1.0 micron.

In embodiments, the tape casting process begins by making an aqueousgarnet slip. The aqueous slip contains DI water, water soluble organicbinder, a plasticizer, and a lithium garnet powder. The solid loadingand binder content in the slip can be varied for achieving a variety ofhigh quality green tapes. After removing substantially all water, thegarnet powder in the resulting dry tape is over 60 vol %. The dry tapethickness can be, for example, of from 10 to 150 microns depending onthe casting equipment and conditions selected. Subsequent lamination iscalled for to make a desirable membrane thickness, which can be, forexample, 20 to 300 microns after sintering. The lamination condition istypically accomplished, for example, at 60 to 80° C. and 1000 psi for 20mins or longer.

In embodiments, the tape can be sintered on a particular garnet setter.The setter has two functions: first, the setter is preferably anon-reactive support; and second, the setter can compensate for lithiumloss during sintering in air. Some garnet compositions may not need thesecond Li compensation function. However, the first function is calledfor to sinter a clean garnet tape as an electrolyte. In embodiments, thelithium loss can additionally or alternatively be compensated for byplacing some mother powder proximate to the sintering tape. Both setter(and mother powder, if any) and samples are placed in a closedenvironment, for example, by covering with a platinum crucible. Theclosed environment is significant for preventing lithium loss duringsintering in air. The sintering temperature can depend on, for example,the composition, a temperature of from 1050 to 1250° C. in air, theholding time at top temperature can vary from 2 to 6 hrs depending onthe size of samples. If the sample is sintered in Ar condition (i.e.,pressure-less), the sintering temperature can be reduced by, forexample, 50 to 100° C. compared to the sintering temperature in air.

In embodiments, after sintering, the resulting tapes are dense,translucent, and hermetic, and the tape thickness can be, for example,from 20 to 200 microns, including intermediate values and ranges. Inembodiments, the ion conductivity of the sintered tapes can be over2.0×10⁻⁴ S/cm.

The method of the present disclosure is advantaged is several aspects,including for example:

Process: Garnet powder can be easily achieved by solid state reaction.The cubic garnet phases can be stabilized by doping alumina, niobiumoxide, gallium oxide, tantalum oxide or other elements. Garnet settersare produced by traditional ceramic forming methods, such as uni-axialpress, iso-press, calendaring, or extrusion. Garnet membrane is formedby tape casting from aqueous slip system with single dried tapethickness from 10 to 150 microns. The lamination of 2 to 6 layers can beaccomplished, for example, at a temperature of from 20 to 80° C. and apressure of 1000 psi for over 20 minutes. Sintering in air can have atop temperature below 1250° C. For sintering in inert atmosphere, suchas Ar or N₂, a mother powder may not be necessary, and the top sinteringtemperature can be reduced (i.e., lowered) compared to the sintering inair. The membrane thickness can be adjusted by single casting, doublecasting, multi-layer casting, and lamination.

In embodiments, the present disclosure provides an electrochemicaldevice comprising: a negative electrode; a positive electrode; and aninterposed solid electrolyte material, wherein the interposed solidelectrolyte material comprises the abovementioned composite ceramic ofand at least one grain growth inhibitor comprising magnesia in an amountof from 1 to 9 wt. % based on the total weight of the solid electrolyte.

In embodiments, the work of the present disclosure discovered that theLi-garnet oxides readily form super-size grains during the sinteringprocess, for example, more than 100 microns in diameter in amicrostructure having an average grain size of under 10 microns, whichsuper-size grains can degrade mechanical properties of the compositeceramic.

In embodiments, the present disclosure provides an economical processfor making improved ceramic microstructures and enhancing the mechanicalproperties of the ceramic microstructures while maintaining requisiteconductivity. In embodiments, the disclosure provides a compositeLi-garnet electrolyte having a second phase additive, which improves themechanical properties of the garnet-type electrolyte.

High conductivity solid electrolytes offer new design opportunities fornext generation high energy density lithium-ion batteries. Ceramicelectrolytes enable independent design of anode and cathode as theyprovide a hermetic barrier which prevents direct contact of anode andcathode chemistries. A significant aspect for next generation batteriesis to have high lithium ion conducting solid electrolytes, that aresafe, non-flammable, have high thermal stability and reliability, highion conductivity, and prevent polysulfide formation for Li—S cells. Manysolid electrolytes provide either high ionic conductivity or highelectrochemical stability against lithium metal, but not both, such asLAGP or LATP solid electrolytes (see C. J. Leo, et al., “Lithiumconducting glass ceramic with Nasicon structure”, Materials ResearchBulletin, 37 (2002) 1419-1430). Lithium garnets are attractive due totheir high lithium ion conduction and chemical stability in moisture andin air compared to metallic lithium, and their potential as solidelectrolyte for all-solid-state rechargeable lithium batteries (see V.Thangadurai, et al., J. Am. Ceram. Soc, 2003, 86, p 437).Lithium-containing Garnets, such as Li₅La₃M₂O₁₂ (M=Ta, Nb),Li₇La₃Zr₂O₁₂, etc., can accommodate a greater concentration of Li⁺cations in the [La₃M₂O₁₂]⁵⁻ framework, the five lithium cations canoccupy any of these interstitial sites: 3 tetrahedral sites; 6octahedral sites; and 3 trigonal prismatic sites (see E. J. Cuessen,“The structure of lithium garnet: cation disorder and clustering in anew family of fast Li⁺ conductors.”, Chem. Commun., 2006, 412-413). Thekey points in making high ion conductive lithium garnet is to produce acubic garnet phase that provides Li⁺ accommodation and mobilityproperties. Fortunately, many researchers have found ways to makepowders or pellets with cubic lithium garnet phase identified by X-raydiffraction (see R. Murugan, et al., “Fast Lithium Ion Conduction inGarnet-Type Li₇La₃Zr₂O₁₂ ”, Angew. Chem. Inst. Ed, 2007, 46, p7778-7781; and Geiger, C., et. al., “Crystal Chemistry and Stability of“Li₇La₃Zr₂O₁₂” Garnet: A Fast Lithium-ion Conductor”, Inorg. Chem.,2011, 50, p 1089-1097.) A thin membrane (i.e., thickness less than 0.5mm), although experimentally difficult to make has been reported thathas practical use as solid electrolyte. A thin membrane made by tapecasting has also been reported (see US Patent Pub 2015/0099188, “Garnetmaterials for Li secondary batteries and methods of making and usingGarnet materials”; and US Patent Pub 2014/0287305, entitled “Ionconducting batteries with solid state electrolyte materials”).

In embodiments, the present disclosure provides a composite Li-garnetoxide ceramic. The ceramic contains the Li-garnet oxides and a secondadditive. The phases of the ceramic are comprised of a cubic garnetphase and a composite phase.

In embodiments, the present disclosure provides a garnet oxide in aceramic that comprises or includes at least one of a known garnet-typeoxide of the formula Li_(6.4)La₃Zr_(1.4)Ta_(0.6)O₁₂ and can beapplicable to other garnet-type oxide groups, for example:

(i) cation substitutions of Li₇La₃Zr₂O₁₂ in the formula ofLi_(7-x)La₃(Zr_(2-x)M_(x))O₁₂ wherein M is, for example, Al, Ga, In, Si,Ge, Sn, Sb, Bi, Sc, Y, Ti, Hf, V, Nb, Ta, and W;

(ii) other lithium ionic conductors having a garnet-type structure; and

(iii) a mixture of the garnet oxides described in (i) and (ii).

Janani describes an improvement in densification that can be achieved byadding sintering aids. Lithium phosphate, lithium borate, and lithiumsilicate are tested as sintering aids, all at a level of 1 wt %addition, and reports higher density and conductivity for thecompositions made with sintering aids. This is different from thepresent disclosure in several respects. First, the present disclosureadds more material, to make a “composite” and not a mere doped garnet.At a 1 wt % addition, conventional terminology would describe theresultant material as a nearly pure garnet, and not a composite. Therole of the additive is to increase density by introducing a modifiedliquid phase during the sintering process that either lowers therequired sintering temperature or makes sintering more effective at agiven temperature. More effective sintering means a higher density orless porosity in the product. Furthermore, additives used in Janani donot restrain grain growth, as does MgO in the present disclosure. It isapparent from the SEM's in the Janani article that grain size increaseswith the included additives, and is not decreased as was discovered inthe present disclosure (see Janani, et al., RSC Adv., 2014, 4, 51228).

In embodiments, the disclosed garnet oxides in the ceramic can have, forexample, the cubic garnet crystal structure of the 1a3d space group, ora cubic garnet crystal structure mixing with a small amount such as 15%,preferably less than 10%, of a tetragonal garnet crystal structure(having the I4_(1acd) space group). The total conductivity of thedisclosed garnet oxides can preferably be 1×10⁻⁴ S/cm or more.

In embodiments, the additive can include, for example, MgO, and alsoCaO, ZrO₂, HfO₂, and like oxides, or mixture thereof. In embodiments,the additive can include, for example, carbonates, sulfonates, nitrates,oxalates, chlorides, fluorides, hydroxides along with the other elementsin the chemical formula. The resulting oxide content of the additivematerials can be, for example from 1 to 9 wt %, from 1.25 to 8 wt %,from 1.5 to 7 wt %, from 1.75 to 6 wt %, from 2 to 5 wt %, from 2.25 to4.5 wt %, from 2.5 to 4 wt %, from 2.75 to 3.5 wt %, and like amounts,including intermediate values and ranges, based on the 100 wt % total ofthe composite comprising the garnet phase material and the grain growthinhibiting second additive.

The second additive component can improve the microstructure uniformityof the sintered garnet electrolyte. In embodiments, the grain sizedistributions in the sintered garnet oxide ceramics can be, for example,decreased to less than an average of 10 microns, and the abnormal growthof the garnet grains at the high sintering temperature can be, forexample, significantly restrained.

In addition, the fracture strength of the disclosed Li-garnet compositesintered electrolyte can be, for example, higher than 100 MPa (testingby the three-point bending technique, geometry: rectangle, sample size:3×4×30 mm). A Li-garnet composite electrolyte as disclosed herein,having improved strength can facilitate, for example, the fabrication ofLi—S, Li-Air, and all-solid-state lithium secondary batteries.

According to an exemplary method of making the disclosed compositegarnet electrolyte, the relative density of the Li-garnet compositesintered electrolytes can be, for example, greater than 95% such as from96 to 99%, from 97 to 98%, including intermediate values and ranges.High relative density of the electrolytes will restrain, for example,lithium dendrite formation and liquid electrolyte permeation into theLi—S, Li-Air and all-solid-state lithium secondary batteries constructedwith the disclosed garnet composite electrolyte.

In embodiments, the lithium ion conductivity of the sinteredelectrolytes can be, for example, maintained above 1×10⁻⁴ S/cm, and theactivation energy (E_(a)) can be, for example, lower than 0.4 eV. Theuse of the Li-garnet composite electrolytes in the Li—S, Li-Air, andall-solid-state lithium secondary batteries can permit the lithium ionsto be more readily conducted, which can decrease the inner resistanceand increase the rate of charge and discharge. The use of the disclosedLi-garnet composite electrolytes in these batteries also broadens theirworking temperature ranges.

In embodiments, in an exemplary method for making the disclosed garnetoxides having at least one second additives can include, for example,the steps:

a first mixing of inorganic starting batch materials to form a mixture,including a lithium source compound, and other inorganic precursors orsource materials to make the desired garnet composition, and thenmilling the mixture to reduce the particle size to, for example, of from0.3 to 4.0 microns (particle sizes in one slurry after the first millingprocess) of the precursors;

calcining the milled mixture to form a garnet oxide, for example, atfrom 800 to 1200° C.;

a second mixing of the garnet oxide and a second additive to provide asecond mixture, and then a second milling of the second mixture toreduce the particle size to, for example, of from 0.15 to 2.0 microns(e.g., particle sizes in one slurry after the second milling process) ofconstituents of the second mixture; and

compacting the milled second mixture into a compact; and

sintering the compact, at for example, from 1050 to 1280° C.

Each of the foregoing steps is described in further detail below.

The First Mixing Step

In the first mixing step, a stoichiometric amount of inorganic materialsare mixed together, in the formula of garnet oxides and, for example,milled into fine powder. The inorganic materials can be, for example, acarbonate, a sulfonate, a nitrate, an oxalate, an hydroxide, an oxide,or mixtures thereof with the other elements in the chemical formula.

In embodiments, it may be desirable to include an excess of a lithiumsource material in the starting inorganic batch materials to compensatefor the loss of lithium during the high temperature of from 1050 to1280° C. (e.g., 1150° C.) sintering step. The first mixing step can be adry milling process, or a wet milling process with an appropriate liquid(i.e., a non-solvent) that does not dissolve the inorganic materials.The mixing time, such as from several minutes to several hours, can beadjusted, for example, according to the scale or extent of the observedmixing performance. The milling can be achieved by, for example, aplanetary mill, an attritor, or like mixing or milling apparatus.

The Calcining Step

In the calcining step, the mixture of inorganic material, after themixing step, is calcined at a predetermined temperature, for example, atfrom 800 to 1200° C., including intermediate values and ranges, to reactand form the target Li-garnet oxides. The predetermined depends on thetype of the garnet oxides. The calcination time, for example, from 3 to12 hrs (e.g., 6 hrs), can also depend upon on the relative reactionrates of the selected inorganic starting or source batch materials. Inembodiments, a pre-mix of inorganic batch materials can be milled andthen calcinated or calcined, as needed, in a first step.

The Second Mixing Step

The calcined Li-garnet oxide mixture and the second phase additives aremixed together and ground to form a mixture of a homogeneous composition(e.g., as determined by the MgO distribution in green ceramic pellets orbars). The second mixing step can include, for example, one or more of:a wet-milling; a dry-milling; or a combination thereof. During millingof the mixture, one can optionally heat the mixture at a low temperatureof, for example, from 60 to 100° C. (e.g., 70° C.) to remove adsorbedmoisture or solvents.

The Compacting Step

The homogeneous second mixture composition was pulverized simultaneouslyduring the second mixing step and compacted to form a compact. Thecompact was sintered at a temperature higher than the temperature of thecalcining step to get dense ceramic pellets. The compact can be formedas arbitrary shapes by any suitable method, for example, cold isotropicpressing, hot isotropic pressing, hot pressing, or by like means andinstrumentalities.

The Sintering Step

During the sintering step, the compact was optionally covered by amother powder to prevent the loss of volatile components. The sinteringtemperature was, for example, from 1000 to 1300° C., includingintermediate values and ranges.

EXAMPLES

The following Examples demonstrate making, use, and analysis of thedisclosed ceramics and methods in accordance with the above generalprocedures.

Comparative Example 1

Comparative Example 1 was accomplished as described in Examples 2 to 10with the exception that the wt % MgO was 0 wt %, i.e., free of any Mg orMgO.

Examples 2 to 10

The Examples 2 to 10 demonstrate the preparation of garnet-type oxidesof the formula “Li_(6.4)La₃Ta_(0.6)Zr_(1.4)O₁₂ wt % MgO” (see Table 1),wherein the Mg²⁺ ion could not go into the lattice of LLZTO and occupyLa³⁺ 24 c site. The garnet-type oxides were synthesized as follows: agarnet-type oxide Ta-doped Li₇La₃Zr₂O₁₂ was prepared via a conventionalsolid-state reaction method, having the formulaLi_(6.4)La₃Ta_(0.6)Zr_(1.4)O₁₂ (LLTZO). LiOH H₂O (ACS Analytical Reagentgrade; “AR”), ZrO₂ (AR), La₂O₃ (99.99%), and Ta₂O₅ (99.99%) followingthe stoichiometry ratio of the desired empirical formula were mixedtogether by a wet grinding process with isopropanol and zirconia ballsused as the milling media, while 2 wt % excess of the lithium sourcecompound was added to compensate for subsequent lithium evaporation athigh sintering temperatures. The reaction of the mixture of reactantswas accomplished by twice calcining at 950° C. for 6 hr for improvingthe uniformity of the synthesis powder. Finally, the synthesized powder,having from 0 to 9 wt. % added MgO, was ground again to a fine powder bya wet grinding with isopropanol and zirconia balls as the milling media.The mixture was dried at 80° C. for 16 hrs and then heated at 500° C.for 1 hr to remove absorbed moisture and solvents. The dried mixture wasthen dry-milled to produce a homogeneous fine powder. After the firstmilling step, the powder was pressed (i.e., compacted) into arectangular bar (about 5×6×50 mm) under the pressure of 200 MPa by thecold isotropic method. Then the compact was covered by the same motherpowder composition and finally sintered at 1250° C. for 10 hr in aplatinum crucible. The sintered samples were polished with diamondpaper. The final dimensions of the polished sample specimens were about3×4×30 mm.

The measured properties of garnet-type oxides of Comparative Example 1and Examples 2 to 10 were as follows:

Phase Analysis

Powder X-ray diffraction (PXDR) (Rigaku, Ultima IV, nickel-filteredCu-Kα radiation, λ=1.542 Å) was employed to determine the phaseformation of the synthesis powder or pellets at about 25° C. in the 20range of from 10 to 800 with a step of 0.1°/sec.

Referring to the Figures, FIG. 1 shows the XRD patterns of the sinteredsamples of Comparative Example 1, and Examples 2 to 10, which weresynthesized via a solid-state reaction method. All the well-defineddiffraction peaks can be indexed to a cubic garnet-type oxide (LLZO),with positions and relative intensities coinciding with a calculationbased on the reference structure (see Geiger, C. A., supra.). A weakpeak matched with crystal plane (200) of MgO cubic phase on the leftshoulder of the peak was indexed to diffraction plane (532) of cubicgarnet phase. Except for this peak, no other impurity was found in theXRD patterns. The intensity of the peak gradually increased with the MgOcontent in the garnet. But the garnet peaks did not shift aftersintering with MgO even in the pattern of Example 10, indicating thatMgO was stable with the garnet oxides or only a very small amount (i.e.,much less than 1 wt. %, such as 0.01 to 0.1 wt. %) can get into thegarnet structure.

Conductivity and Activation Energy (E_(a))

The ionic conductivity was measured at room temperature by the ACimpedance analysis (Autolab, Model PGSTAT302N). The potentiostaticimpedance model of frequency resistance analysis was selected as thetest model, while the frequency ranged within 1 Hz to 1 MHz, and theelectrical perturbation was set to 20 mV. Before the measurement, boththe parallel surfaces were sputtered with gold as the lithium ionblocking electrode. The measurement results are shown the FIG. 2 and therelationships between the total conductivity and the MgO content isshown in FIG. 4. FIG. 2 shows AC impedance measurement at roomtemperature corresponding to Control Example 1 and Examples 2 to 10,respectively. The total conductivity of each of the samples of ControlExample 1 and Examples 2 to 10 was more than 1×10⁻⁴ S/cm at roomtemperature. The highest values corresponded to the composite samples ofExamples 4 to 6 and were more than 5×10⁻⁴ S/cm at room temperature. FIG.4 is a graph showing the relationships between MgO content in weightpercentage (wt %) and the total conductivity (lower curve and rightaxis), and MgO content in weight percent and the activation energy(E_(a)) (upper curve and left axis) corresponding to each of the ControlExample 1 and Examples 2 to 10, respectively, and listed in Table 1.

The activation energy (E_(a)) was measured at from 298 to 388° K in atemperature chamber and calculated from the slope of an Arrhenius plotaccording to the equation:

αT=A exp(E _(a) /kT),

wherein σ is the conductivity, A is the frequency factor, E_(a) is theactivation energy, k is the Boltzmann constant, and T is the absolutetemperature. The activation energies of Control Example 1 and Examples 2to 10, were from 0.36 to 0.38 eV, which was obtained by the linearfitting of the Arrhenius plot shown in the FIG. 3. The relationshipbetween the activation energy and the MgO content is shown in the FIG.4, indicating that the second additives did not have an apparent effecton the activation energy of the Li-garnet oxides.

Relative Density

The density of the specimens was tested by the Archimedes method withthe Mettler-Toledo density measurement attachments. The theoreticaldensity was calculated from the XRD results in the FIG. 1. FIG. 1 is agraph showing XRD patterns of Control Example 1 and Examples 2 to 10.The vertical lines (e.g., 211, 321, 400, 420, 422, 521, 532, 640, 800)at the bottom represent reference data (see Geiger, C. A., infra.), andthe vertical line (e.g., 200) at the bottom were MgO data from standardcards. The practical density, theoretical density, and relative densityof Comparative Example 1 and Examples 2 to 10 is shown in FIG. 5. Whenthe MgO content was more than 3 wt. %, the relative density of thesintered samples was more than 96%. FIG. 5 shows the relationshipsbetween MgO content (in wt %) of the disclosed ceramics and thetheoretical density (left axis), the practical or real density (leftaxis), and the relative density (right axis) corresponding to ControlExample 1 and Examples 2 to 10, respectively.

Fracture Strength and Vickers Hardness

The mechanical strength of Comparative Example 1 and Examples 2 to 10was determined by the three-point bending technique (Instron 3366). Thebottom spans were 20 mm and the loading rate was 0.02 mm/min. TheVickers hardness, H, was measured with a hardness tester. The averagehardness of Comparative Example 1 and Examples 2 to 10 was determinedusing five indentations per load on the polished surface at the loads of4.9 N with a 5 s loading time. To avoid the interference between stressfields of the closely spaced indentations or perturbations caused byspecimen edge effects, separation distances of at least 500 microns weremaintained between adjacent indentation sites. The fracture strength andVickers hardness measuring results are shown in FIG. 6. When the MgOcontent was more than 3%, the fracture strength was higher than the pureLi-garnet oxides.

Pristine and Chemical Corrosion Cross-Section Microstructure

The random cross-section microstructure morphologies of sintered samplesof Control Example 1 and Examples 2 to 10 were revealed by the SEM(Hitachi, S-3400N). The random cross-section microstructures are shownin FIGS. 7A to 7J, respectively. These sample were chemically etchedwith an aqueous 6 M HCl solution for 3 min to reveal the grainboundaries and the chemical corrosion morphologies are shown in FIGS. 8Ato 8J, respectively (i.e., where image: a) Control Ex 1=0 wt %, b) Ex2=1 wt %, c) Ex 3=2 wt %, d) Ex 4=3 wt %, e) Ex 5=4 wt %, f) Ex 6=5 wt%, g) Ex 7=6 wt %, h) Ex 8=7 wt %, i) Ex 9=8 wt %, and j) Ex 10=9 wt %).When the MgO content was more than 3 wt %, the grain size of theLi-garnet oxides was significantly decreased to less than 10 microns andan appropriate dense microstructure was obtained. The overgrowth of thegarnet grains was notably restrained by the addition of the MgOadditive, and contrary to the reported grain size increase trend (seeJanini supra.).

Distribution of the Second Additive MgO

The Mg distribution in the cross-section of Comparative Example 1 andExamples 2 to 10 was probed by scanning electron microscopy with theenergy dispersive detector (Hitachi, S-3400N). The Mg element mappingscanning images and the backscatter scanning electron images of therandom cross-section are shown in FIG. 9 and FIG. 10, respectively.

FIGS. 9A to 9I show energy dispersive spectroscopy (EDS) images of MgOdistributions of pristine samples corresponding to Examples 2 to 10,respectively (Control Example 1 image not shown). The framed regionsrepresent the detected area for element analysis, and the cross-hatchedshaded areas (red in original color images not included) representexamples of significant (but not all) Mg rich regions. FIGS. 10A to 10Jshow back scattering electrons (BSE) images of pristine samplescorresponding to Control Example 1 and Examples 2 to 10, respectively.

It was noted that the MgO grains were formed in all the compositeceramic electrolytes. When the MgO content was more than 3 wt %, MgOgrains existed between the garnet oxides grains and restrained thegrowth of garnet grains. The MgO content of more than 3 wt % alsoallowed a very small amount of Mg into the garnet structure, which wasnot detected by the energy dispersive detector. Table 1 lists the SEManalysis of Comparative Example 1 and Examples 2 to 10.

TABLE 1 Summary of SEM Analysis of Control Example 1 and Examples 2 to10. Composition Li_(6.4)La₃Ta_(0.6)Zr_(1.4)O₁₂ + Activation FractureVickers Example wt % (mol %) MgO Conductivity energy Relative strengthhardness No. additive (vs. LLZTO) (10⁻⁴ S/cm) (eV) density (MPa) (GPa) 1(Control) 0 wt % (0 mol %) 6.7 0.366 97% 92.5 6.38 2 1% (22%) 3.2 0.38095% 60.4 7.00 3 2% (44%) 4.5 0.365 96% 77.2 7.77 4 3% (66%) 5.7 0.38198% 100 7.61 5 4% (88%) 4.8 0.367 98% 117 7.96 6 5% (110%) 5.2 0.362 98%135 7.46 7 6% (132%) 3.75 0.373 97% 132 7.47 8 7% (154%) 4.25 0.364 97%122 7.30 9 8% (176%) 3.41 0.363 97% 132 7.11 10 9% (198%) 4.36 0.360 96%138 7.04

Lithium garnet has garnet-like structure and predominantly ionicconduction (see Cuessen, E. J., “The structure of lithium garnet: cationdisorder and clustering in a new family of fast Li⁺ conductors”, Chem.Commun., 2006, p 412-413) with a nominal chemical compositionLi₇La₃Zr₂O₁₂. This composition is reported to have high lithium ionconductivity, and good thermal and chemical stability against reactionswith a lithium metal electrode. The composition is easy to prepare fromlow cost materials and is readily densified at low temperatures around1200° C. These features suggest that zirconia-containing lithium garnetis a promising candidate for a solid electrolyte for lithium battery.

The garnet structure of Li₇La₃Zr₂O₁₂ can possess two crystalline phases:tetragonal and cubic. The tetragonal phase is a stable phase at ambientcondition, but it may undergo a phase transition to the cubic phasebetween 100 and 150° C. (see Geiger, C., supra.). The tetragonal garnethas space group symmetry I4₁/acd and is characterized by lower ionconductivity compared to cubic garnet (see Awaka, J., et al., J. SolidState Chem., 2009, 182, 2046-2052). Although the reason for the lowerion conductivity difference is not understood, the researchers werefocused on making stable cubic garnet by exploring additives that wouldeasily maximize the ion conductivity. The typical additives mentioned inthe literature (see R. Murugan, et al., “Fast Lithium Ion Conduction inGarnet-Type Li₇La₃Zr₂O₁₂ ”, Angew. Chem. Inst. Ed, 2007, 46, 7778-7781;C. Geiger, et al., “Crystal Chemistry and Stability of “Li₇La₃Zr₂O₁₂”Garnet: A Fast Lithium-ion Conductor”, Inorg. Chem., 2011, 50,1089-1097) include, for example, Al³⁺, Nb⁵⁺, Ta⁵⁺, Ga³⁺, Sn⁵⁺, Sn³⁺, andlike ions as needed to produce a cubic garnet powder or pellets (seeU.S. Pat. No. 8,986,895, “Garnet type lithium ion-conducting oxides andall solid state lithium ion secondary battery conducting the same,” S.Ohta, et. al.). However, there are few reports on making lithium garnetmembranes by tape casting since several challenges limit this approach.First, tape casting involves solvents and binders, which can lower thecompaction of green body. Second, tape casting is a pressure-lessforming method, and the green density is lower than pressure formingmethods that most prior work has reported. Third, the lithium garnetmembrane is extremely thin, so it is not practical to cover the tape bymother powder as most researchers have done for thicker pellets. Fourth,the lithium garnet or its precursors are chemically reactive and canreadily react with other materials, so it is challenging to identifysuitable setter materials. To overcome the above challenges, newcompositions and improved processes have been developed and arepresently disclosed.

In embodiments, the disclosure provides a method of making lithiumgarnet that begins with a garnet powder fabrication. A batch includingoxide precursors is thoroughly mixed. The final composition aftercalcination has a formula: Li_(7-x)La_(3-y)Zr_(2-x)A_(x)B_(y)O₁₂. Toensure homogeneity, the starting batch powder can be dry-mixed byTurbula® Shaker mixing, for example, for at least 30 minutes, vibratorymixing at least 30 min, and ball milling over 2 hr. Wet mixing isrecommended but not required. The wet mixing can be conducted in, e.g.,water or isopropyl alcohol with a vibratory mixer for 2 hrs to breakdown the agglomerates, then ball-milled on a roller for 1 to 2 hrs, thendried at 100° C. for 1 to 2 days. The dried and mixed powder wascalcined in a platinum container covered by an alumina sheet at, forexample, from 1000 to 1250° C. for 2 to 12 hrs. The optimal calcinationtemperature will depend on the particular garnet composition selected.After calcination, the powder was milled by either ball milling or jetmilling with 90 wt. % of the above lithium garnet cubic phase. The ballmilled powder was coarser, having particles having a D50 of from 1 to 5microns, the jet-milled powder was finer having a D50 from 0.5 to 0.9microns. Both the coarse and fine powders have approximately a bi-modalparticle size distribution. For tape casting, a finer powder having amono-modal distribution is preferred. A mono-modal fine powder, e.g., 3microns, can be achieved by air classification of the ball milledpowder. Three powders were produced after air classification and theirparticle size distributions are shown in FIGS. 11A and 11B. Theultrafine powder (e.g., 0.3 to 0.4 microns) is a preferred garnet powderfor tape casting. The “−3 micron” (e.g. circa 0.7 microns) type powderis suitable for some compositions. The “+3 micron” (e.g., 4 to 5.5microns) powder is more difficult to make as a hermetic tape. Theultrafine powders have a mono-modal distribution, which gives high greencompaction after tape casting, and uniform shrinkage that ensures theflatness of green tape and sintered tape.

FIGS. 11A and 11B, respectively, show particle size distributions by vol% for a Ga—Nb doped garnet composition of the formulaLi_(6.75)La_(2.8)Ga_(0.2)Zr_(1.75) Nb_(0.25)O₁₂ (“HF 110”), and for aTa-doped garnet composition of the formulaLi_(6.5)La₃Zr_(1.5)Ta_(0.5)O₁₂ (“HQ 110”). In each instance, the garnetpowders were ball-milled and then air classified into three fractions:“ultrafine,” “−3 microns” and “+3 microns.” For HF 110 d50 of theultrafine fraction is 0.4 microns (1110); d50 of the −3 micron fractionis 0.71 microns (1120); and d50 of the “+3 micron” fraction is 4.0microns (1130). For HQ 110, d50 of the ultrafine fraction is 0.4 microns(1130); d50 of the −3 micron fraction is 0.7 microns (1140); and d50 ofthe “+3 micron” fraction is 5.3 microns.

The tape casting process includes, for example, slip making, tapecasting, drying, and lamination. An aqueous slip has been formulatedherein for making garnet tape. Garnet powder is reported to be morestable in acid and basic aqueous solution than LTAP (see Y. Shimonishi,et. al, “Synthesis of garnet-type Li_(7-x)La₃Zr₂O_(12-1/2x) and itsstability in aqueous solutions”, Solid State Ionics, 183 (2011) 48-53).Their tests were conducted at 50° C. for one week. In the presentlydisclosed method, stability tests of the powder or the pellet in wateralso show a reaction of garnet with water at room temperature. Theobjective is to tape cast within relatively short time after slippreparation and prior to any substantial change of slip rheology. Atypical aqueous slip composition formulation is listed in Table 2. Thesupport was Mylar 05270, and the applicator blade had a 14 mil gapwidth. Table 3 shows the information of aqueous binders used in theslips. The slip making includes the steps of: dispersing the garnetpowder in de-ionized water to form a garnet suspension; adding binder tothe garnet suspension; high speed mixing with vacuum and chilling for 5to 10 min; and tape casting using a 6 mil to 18 mil blade. The tape is90 wt % dried in less than about 24 hrs (e.g., 10 wt % residual water),then it is laminated at 60 to 80° C. at 1000 psi for 20 min. Thelaminated tape has a thickness of from 100 to 400 microns with an areaof, for example, from 1 to 3 square inches or larger.

TABLE 2 Example aqueous slip composition formulation. Density Slip SlipVol Slip Component (g/cm³) (g) wt % (cc) vol % Solvent DI Water 1.030.000 56.60 30.000 73.3 Solution Garnet Garnet 4.5 15.000 28.30 3.3338.1 powder, air classify 0.396 microns Binder & Binder for 1.050 8.0015.1 7.619 18.6 Plasticizer aqueous base (no plasticizer) Total — 53.000100 40.952 100

Table 2 lists three different proprietary water based binderformulations from Polymer Innovations (Vista, Calif.) that were testedfor compatibility with the disclosed garnet powders.

TABLE 3 Water based binder formulations. Water Binder Plasticizer BinderContent Content Content Name (wt %) (wt %) (wt %) WB4101 65 28 7WB40B-44 65 30 5 WB40B-53 74 26 0

The binder can include, for example, an aqueous solution of aproprietary acrylic polymer having copolymerized polar functional groups(according to the commercial source: Polymer Innovations, seepolymerinnovations.com). The acrylic polymer has a T_(g) ofapproximately 40° C. but can be plasticized to a much lower T_(g) topermit typical tough cast tape properties. The polymer is soluble inwater when the pH is alkaline, and can be achieved by, for example, theaddition of a small percentage (e.g., 0.1 to 3 wt %) of ammoniahydroxide.

It was determined that the WB4101 and WB40B-44 binders systems, whichalso contain plasticizer, adversely affected the rheology of the slurry.This resulted in flocculation and an inability to cast a suitable tapefor some compositions. The WB40B-53 binder, free of added plasticizer,allowed for a slurry with high viscosity that could be cast to a thinworkable tape for sample preparation, if cast immediately after mixing.In embodiments, WB40B-53 is a preferred binder for preparing thepresently disclosed garnet tapes.

Sintering of laminated tape in air calls for several conditions. First,the tape is placed on a garnet setter, each garnet composition may needits own garnet setter of the same or similar composition. The setterpreferably has a higher sintering temperature than the tape so that thetape does not stick to the setter during sintering. The setter is alsopreferably a composition that is rich in lithium, which providesadditional Li for the tape if needed. Second, the mother powder ispreferably the same composition as the garnet powder when the tape isplaced around the setter or near the tape to compensate for the loss ofLi in the tape during sintering. A mother powder may not necessary forall garnet compositions. The need for a mother powder can depend on thecomposition. For example, in a lithium rich composition, a mother powderis not needed. For a lithium deficient composition, the tape may need tobe buried in a mother powder. A preferred Li content in the empiricalformula composition can have a stoichiometry of, for example, from 6.4to 6.8 mol equivalents (i.e., Li_(6.4 to 6.8)). Third, a closedenvironment is necessary to enclose the tape, the setter, and the motherpowder such as in an inverted platinum crucible firing apparatus (1400)shown in FIG. 14. The bottom support is a platinum sheet, the garnetsetter is placed on top of the sheet along with the mother powder, andthe platinum crucible contacts the bottom platinum sheet. At lowtemperature, the gap between the crucible and sheet can release evolvedgas, while at high temperature (greater than 800° C.), the gap is closedbecause of a weight (e.g., Al₂O₃ (1440)) on the top of platinumcrucible. Such closed system makes the sintering of garnet tape veryeffective. Fourth, the sintering in air is preferably accomplished in aregular kiln. A slow heating rate below 400° C. is preferred such asfrom 60 to 90° C./hr due to the binder burn out. Above 400° C., theheating rate can be, for example, above 100° C./hr, such as set at 120°C./hr, until at the top sintering temperature. The top sinteringtemperature depends on the composition, and can vary, for example, from1000 to 1250° C. The dwell time on the sintering temperature can be, forexample, from 2 to 10 hrs depending on the composition, the mass ofsample, and the grain size target.

For sintering in an inert gas environment, the green tape (1410) can beplaced directly on a platinum support (e.g., a plate (1420), oralternatively, on a garnet setter (1430) depending on the composition(see FIG. 14). The tape was covered by an inverted platinum crucible(1440) to reduce the loss of lithium, but the sealing does not need tobe as tight as when sintering in air. The objective is to begin thefiring in air to burn out the binder or other organics, usually up to800° C. to remove all carbon, then changing to an inert atmosphere, suchas Ar or N₂, for the remainder of the firing. The inert gas flow ispreferably maintained at, for example, 40 cfph throughout the inert gassintering phase. The top sintering temperature can be lower for samplessintered in air, and can be, for example, at least 50° C. lower thansamples sintered in an inert atmosphere. The tapes sintered in an inertatmosphere (i.e., oxygen free) are also dense, translucent, hermetic,and have the same ion conductivity as tape samples sintered in air. FIG.15 shows an example of a tape sintered in an inert atmosphere.

Example 11

Making a Lithium Garnet Setter The lithium garnet setter is an aluminumdoped composition of the formula Li_(6.1)La₃Zr₂Al_(0.3)O₁₂. Thiscomposition has a high sintering temperature about 1240° C. in airhaving a lithium garnet cubic phase of over 95 wt % or 95 vol %, so thislithium garnet setter is a good firing support material for a lithiumgarnet tape. The setters can be made by dry press including uni-axialpress at about 2000 psi and iso-press at about 8000 psi. The formedgreen pellet, usually 2 to 3 inches (50.8 mm to 76.2 mm) in diameter and1 to 3 inches (25.4 mm to 76.2 mm) in height, is placed on a platinumsheet and covered by an inverted platinum crucible. The closedenvironment minimizes the loss of lithium during sintering. The topsintering temperature is 1230° C. for 2 to 6 hr. Since the setter is notnecessary for full densification, the sintering condition can be lowerwith a shorter time holding. After sintering, the resulting sinteredpellet can be cut into disks having a thickness from 0.5 to 5 mm for useas setters. The surface of the setter is preferably flat and smooth, andthe setter area is preferably larger than the size of tape sample placedupon the setter. The setter can be repeatedly used for the disclosed lowtemperature garnet tape preparation.

Another suitable setter is a Ta doped garnet with a composition of theformula Li_(6.5)La₃Zr_(1.5)Ta_(0.5)O₁₂ including 10 wt % of Li₂O addedto the batch. The batch was sintered similar to the abovementioned Aldoped setter and used a sintering temperature of 1250° C. for 4 hrs in aPt crucible. In the tape sintering process, the setter serves as a flatand smooth support for garnet tape but also prevents side reactions ofPt plate with tape. The setter also provides a source of excess Li forgarnet tape to compensate for possible Li loss from the tape by Li₂Oevaporation.

Examples 12

Garnet Tape Sintered inAir-Li_(6.75)La_(2.8)Ga_(0.2)Zr_(1.75)Nb_(0.25)O₁₂ The tape is formed bytape casting from an aqueous slip system. The powder is gallium andniobium bi-doped lithium garnet having stoichiometric composition of theformula Li_(6.75)La_(2.8)Zr_(1.75)Ga_(0.2)Nb_(0.25)O₁₂. The steps ofmaking a slip include, for example: dispersing 15 g of lithium garnetpowder (which has been air-classified with mono-modal particle sizedistribution and D50=0.4 microns), in 30 g DI water; stirring with aspatula for from 1 to 2 mins (Water is an excellent liquid to dispersethe garnet powder. The majority of the powder or the agglomerates arerapidly dispersed.); then adding about 8 g of the aqueous soluble binderWB04B-53. Since this binder did not contain plasticizer, it takes longerto dissolve. The stirring by spatula was continued for 10 to 20 minsuntil all the binder is dissolved. Then the container was placed into ahigh speed mixer (Mazerustar mixer) for 5 mins. The mixer had vacuum andchilling capability, so the slip would not be inadvertently warmed, andreduces the potential for reaction between the garnet with other slipcomponents. After mixing, the slip was immediately cast on a Mylarsurface with a blade of 14 mil gap.

The tape was dried in air for about 20 hrs with some degree of cover,the dry tape was about 90 microns thick from a 14 mil blade, so fourtapes were stacked in a parallel configuration, that means the tape topsurface (face to air) and the tape bottom surface was against the Mylar.The stack was laminated at temperature 60° C. at 1000 psi for 20 mins.After lamination, a flat uniformly thin laminated sheet was produced andwas ready for sintering.

The laminated sheet was placed on the lithium garnet setter mentioned inExample 11. Either an Al or a Ta doped setter can be used depending ontape composition. General guidance is to use a high temperaturecomposition as the setter, preferably from 50 to 100° C. higher than thesintering temperature of the targeted composition. A small amount ofmother powder was spread around the setter, then covered with aninverted platinum crucible. An alumina disc or plate was placed atop ofplatinum crucible as a weight to seal the gap between crucible and sheetat high temperature. The laminated sheet was sintered at 1075° C. for 2hrs. Because of the organic content in the tape, the heating rate fromambient to 400° C. was set at a slow rate of 90° C./hr. After burningoff the binder (i.e., removing the binder with heat), the heating ratewas adjusted to 120° C./hr, and the total cycle time is less than 24hrs, such as from 2 to 23 hrs. The resulting sintered tape appearance isshown as the overlay in FIG. 12. The resulting sintered tape was white,translucent, and hermetic (i.e., air-tight or air impermeable). Thissintered tape composition had a high relative density, and excellent ionconductivity. The microstructure analysis of the tape as shown in FIG.13A indicates that the surface of the tape was very dense, and isbelieved to be what is responsible for the tape being hermetic. FIG. 13Bshows the polished internal surface of the tape, which did not have openpores. Instead all pores were closed. Some of the dark spots were notpores but rather were secondary phases that were Zr rich phases. FIG.13C shows the grain size of the garnet after sintering. For thiscomposition, there is an intermediate liquid phase that promotes thesintering and at the same time accelerates the growth of grains. Mostgrains are less than 5 microns, but some grains are up to 20 microns.The ion conductivity of the tape was 1.3×10⁻⁴ S/cm.

Example 13

Garnet Tape Sintered in Air-Li_(6.5)La₃Zr_(1.5)Ta_(0.5)O₁₂ The Ta dopedgarnet tape is formed by tape casting from aqueous slip system. Thepowder is made by doping 0.5 mol equivalents tantalum into the garnet(Li₇La₃Zr₂O₁₂) to form a stoichiometric composition of the formulaLi_(6.5)La₃Zr_(1.5)Ta_(0.5)O₁₂. The XRD analysis shows that the cubicphase is greater than 95 wt %. To make a slip system, 14 g garnet powder(D50 is 0.36 microns and 0.7 microns after air classification in ballmilling, 0.61 microns after jet milling, respectively) was dispersed in18 g DI water and stirred by spatula to mix well. Then 5.25 g aqueoussoluble binder was added into the slip and stirred by magnet stir barfor 1 hr until all of the binder was dissolved. The binder was WB4101(with 7% plasticizer), WB04B-53, or a mixture thereof, and the ratios ofgarnet/water/binder may vary. A vacuum pump was used to remove bubblesfrom the slip, and then the slip was placed in a vacuum Mazerustar mixerto form a homogeneous slip and further remove any bubble. After mixing,the slip was immediately casted on Mylar with a blade having, forexample, a 8 to 16 mil gap. After drying, several layers of green tapecan be laminated together to form a thicker tape. For a betterlamination, warm iso-static press is preferred. The tape edges were cutoff by laser to form a circular tape aiming to avoid edge curling andwrinkling during the tape sintering process.

A suitable sintering configuration is illustrated in FIG. 14. Thelaminated sheet is placed on the Ta doped garnet setter (described inthe Example 11, Ta-doped with excess Li as setter), and Pt plate, andthen covered with Pt crucible, with alumina weight on top to seal thegap between Pt crucible and plate at high temperature. The sinteringprofile was as follows: heating rate is 100° C./hr from ambient (e.g.,25° C.) to 600° C., and hold for 1 hr to remove any organic residues inthe green tape, and after that the temperature continues to rise to thesintering temperature of 1200° C. at the same heating rate, and then ahold for 2 to 3 hrs to sinter the tape, finally the tape is cooled toambient or about 25° C. with a cooling rate of 300° C./hr.

FIG. 14 shows a schematic of an exemplary sintering apparatus includinga closed environment (1400) for sintering a garnet tape (1410), and caninclude, for example, a supporting platinum plate base sheet (1420), aninverted platinum crucible (1440), an alumina disc weight (1450)surmounting the platinum crucible. Contained within the chamber formedby the inverted platinum crucible is a garnet tape (1410) atop a setter(1430), and optionally a mother powder (1450) (not shown) surrounding orburying the garnet tape (1410).

FIG. 15 shows the tape appearance after sintering in air, and that thetape is translucent, uniform in color, and very flat. The capitalletters “MET” printed on a background surface are clearly visiblethrough the disk to a human eye and in an original photograph.

FIGS. 16A to 16D show SEM cross-section images that demonstrate that thegarnet tapes are sintered dense and have small grain sizes (7 micron).FIGS. 16A to 16D illustrate the microstructure of Ta-doped garnetfractured cross sections. FIGS. 16A to 16B (scale bars are 240 and 60microns, respectively) are SEM images for tape made from ball milled andair-classified garnet powder (0.7 microns) as shown in FIG. 11B, andusing binder with plasticizer WB4101. FIGS. 16C to 16D (scale bars are230 and 60 microns, respectively) are images for a tape made from jetmilled garnet powder using binder without plasticizer WB40B-53. Theimage scale bars are 240 microns (FIG. 16A), 60 microns (FIG. 16B), 230microns (FIG. 16C), and 60 microns (FIG. 16D), respectively. FIGS. 16Ato 16D demonstrate that both tapes are sintered dense and have grainsizes that are quite uniform, and less than 15 microns. EDX analysisshows the presence of elements La, Zr, Ta and O (data not shown, Li notdetectable by EDX), which is consistent with the garnet composition. Thetapes are hermetic and they are still intact after 3 months of amethylene blue leak test. The sintered tapes have almost 100 wt % cubicphase by XRD analysis. The conductivities of tapes are high, forexample, having conductivities from 2 to 3×10⁻⁴ S/cm.

Example 14

Garnet Tape Sintered in Ar or N₂ Gas The lithium garnet tapes sinteredin Ar or N₂ are very different from those sintered in air. Theoxygen-free environment not only avoids the reaction between the garnetand platinum, but also lowers the sintering temperature. The tape hasthe same composition Li_(6.75)La_(2.8)Zr_(1.75)Ga_(0.2)Nb_(0.25)O₁₂. Butthe sintering temperature is lower than Example 13, and the tape can besintered at as low as 1030° C., which is at least 40° C. lower. FIG. 17shows a graph of a representative sintering profile of a disclosedgarnet tape that was initially heated in a tube furnace in airatmosphere (1710) to an intermediate temperature, and then in either anAr or N₂ atmosphere (1720) for the remaining part of the profile up tothe final sintering temperature and in cool down to ambient.

The tapes were fired in a tube furnace, which allow control of the gasenvironment. The firing profile was divided into two stages: from roomtemperature to 800° C., the tape was fired in air to eliminate allorganics; and from 800 to 1040° C. and the reminder of the firing, thetape was fired in Ar or N₂ environment. At 800° C., there is a two-hourholding, the gas is switched after one hour holding or at the last 30min, to ensure the air is flowing out and the chamber is filled with Aror N₂ gas before heating to a higher temperature. The sintering at 1040°C. is preferably accomplished in an oxygen-free environment. Aftersintering, the tapes were also translucent. Similar to the Example 2,the grains have growth that is similar to the growth level observed whensintered in air. The ion conductivity of the sintered product isconsistently about 5×10⁻⁴ S/cm.

Another composition Li_(6.5)La₃Zr_(1.5)Ta_(0.5)O₁₂ was also sintered inan Ar atmosphere. The sintering temperature was lower by at least 50° C.compared to the sintering in air. Since this composition without liquidsintering, the sintering temperature in air is equal or above 1200° C.,while in Ar condition, the sintering temperature can be lowered to 1140°C. The temperature profile is similar to FIG. 17 except the toptemperature is higher. After sintering, the tape is very flat,translucent, and hermetic, but the transparency is not as good as thepreceding Ar sintered example, mainly believed to be attributable todifferences in the compositions. The ion conductivity was 3.2×10⁻⁴ S/cm.

Example 15 (Prophetic)

Solid Electrolyte and Energy Storage Device including the Solid A fullbattery cell is fabricated by laminating a thin lithium metal anode toone side of the garnet solid electrolyte and printing or casting acathode layer such as conventional lithium cobalt oxide on the oppositeside. Preferably the cathode structure includes a gel electrolyte suchthat the electrolyte included within the cathode does not migrate. Thinmetal foil current collectors are added to the electrode layers forcurrent distribution to complete one “trilayer” structure. Multipletrilayer structures can be integrated together (i.e., combined) toincrease the cell capacity if desired. With a thin solid electrolyte (20microns thick), a thin Li metal anode (less than 10 microns thick),conventional current collectors (Al/Cu at 15 microns thick), and alithium cobalt oxide/gel electrolyte composite at a capacity of at least3 mAh/cm², a cell with volumetric energy density of greater than 600Wh/L is achievable.

The disclosure has been described with reference to various specificembodiments and techniques. However, it should be understood that manyvariations and modifications are possible while remaining within thescope of the disclosure.

1-17. (canceled)
 18. A composite ceramic comprising: a lithium garnetmajor phase and a grain growth inhibitor minor phase positioned betweengrains of the lithium garnet major phase, of the formula:Li_(7-x)La₃(Zr_(2-x),M_(x))O₁₂—SA, where M is selected from the groupAl, Ga, In, Si, Ge, Sn, Sb, Bi, Sc, Y, Ti, Hf, V, Nb, Ta, W, or amixture thereof; and “SA” comprises a second additive oxide, present infrom 3 to 9 wt % based on the total amount of the ceramic; and x isgreater than 0 and less than
 1. 19. The ceramic of claim 18, wherein theceramic has an average grain size of from 3 to 7 microns.
 20. Theceramic of claim 18, wherein the ceramic has a mechanical strength offrom 100 to 180 MPa.
 21. The ceramic of claim 18, wherein the ceramichas an ion conductivity of from 1×10⁻⁴ S/cm to 6×10⁻⁴ S/cm.
 22. Theceramic of claim 18, wherein the density of the ceramic is at least 95to 98% of a theoretical maximum density of the ceramic.
 23. A ceramicelectrolyte comprising at least the ceramic of claim
 18. 24. A method ofmaking the composite ceramic of claim 18, comprising: a first mixing ofinorganic source materials to form a mixture, including a lithium sourcecompound, and other suitable inorganic source materials to make thedesired garnet composition; a first milling of the mixture to reduce theparticle size of the precursors; calcining the milled mixture to form agarnet oxide at from 800 to 1200° C.; a second mixing of the milled andcalcined garnet oxide and a second additive to provide a second mixture;a second milling of the second mixture to reduce the particle size ofconstituents of the second mixture; compacting the second milled secondmixture into a compact; and sintering the compact at from 600 to 1300°C.
 25. The method of claim 24, wherein the lithium source compound ispresent in a stoichiometric excess.
 26. The method of claim 24, whereinthe sintering is accomplished in air, in an inert atmosphere, or firstin air then in an inert atmosphere.
 27. The method of claim 24, whereinthe sintering is accomplished in air at from 1000 to 1300° C.
 28. Themethod of claim 24, wherein the sintering is accomplished in an inertatmosphere at from 800 to 1200° C.
 29. The method of claim 24, whereinthe particle size of the first milling is from 0.3 to 4 microns and theparticle size of the second milling is from 0.15 to 2 microns.
 30. Anelectrochemical device comprising: a negative electrode; a positiveelectrode; and an interposed solid electrolyte material, wherein theinterposed solid electrolyte material comprises the composite ceramic ofclaim 1 having at least one grain growth inhibitor comprising magnesiain an amount of from 3 to 9 wt. % based on the total weight of the solidelectrolyte.
 31. A composite ceramic comprising: a lithium garnet majorphase and a grain growth inhibitor minor phase positioned between grainsof the lithium garnet major phase, wherein the grain growth inhibitorminor phase is present in from 3 to 9 wt % based on the total amount ofthe ceramic.